1. Technical Field
The present invention pertains to the field of communications networks, in particular to the field of packet transport networks.
2. Description of the Related Art
List of acronyms:BCBus control process CycleBEBEnd-Of-BusL1ISO OSI Stack Protocol Layer 1L2ISO OSI Stack Protocol Layer 2L3ISO OSI Stack Protocol Layer 3LECLocal Exchange CarrierMPLSMulti-Protocol Label Switching, IETF RFC 3032ISPInternet Service ProviderIPInternet Protocol, IPv4: IETF RFC 791, IPv6: IETF FRC 2460POPPoint Of PresencePOSPacket Over SDH/SONETPPPPoint-to-Point Protocol, IETF RFC 1661QoSQuality of ServiceSDHSynchronous Digital Hierarchy, ITU-T Recommendation G.707SONETSynchronous Optical Network, a subset of SDH used in NorthAmerica
To provide deterministic QoS for packet switched data traffic, such IP traffic, the communications service provider needs to be able to guarantee pre-definable connectivity parameters between the communicating sites. A conventional way to provide guaranteed connection bandwidth between two sites is to provide a dedicated Layer 1 or Layer 0 point-to-point connection between the two sites. Such a network architecture based on non-shareable connections is however inefficient for bursty packet traffic, since the bandwidth reserved for the dedicated point-to-point connections is wasted when not used for communication between the two sites at the two ends of the point-to-point connection; none of the point-to-point link bandwidth can be used to carry other potential billable traffic even when not needed by traffic between the two sites connected by the point-to-point link. When guaranteed connectivity is needed among more than two sites, such as POPs of an ISP, the dedicated point-to-point link based network architecture is even more inefficient.
As an example of the inefficiency of the dedicated point-to-point connection based network architecture, consider a case where a group of five POP-routers would need guaranteed interconnectivity. Assume further that each one of the five POP routers has 10 Gbps of aggregate full-duplex traffic forwarding capacity between its local access ports and the other four POPs within the group. However, that 10 Gbps of traffic forwarding capacity pool of a POP router needs to shareable among the traffic from the four other POPs, with any potential momentary capacity breakdown pattern supported. For instance, identifying the five POPs as POP A, B, C, D and E, the egress packet traffic forwarding capacity demand break-down at the POP E at some five millisecond time window could be as per Table 1 below:
TABLE 1An assumed breakdown of the 10 Gbps egress traffic forwarding capacitypool of POP E router among the source POPs A-D over a 5 ms periodsuch that maximizes the billable traffic throughput for POP E egresstraffic.Time/msABCD12.52.5052151330100040.550.5452.52.52.52.5
As illustrated in Table 1 above, in an extreme scenario, the full egress forwarding capacity pool of a POP router may be demanded by a single one of the group of directly interconnected POPs. Thus, in order to not limit the traffic delivery capacity of the network between the five POPs, the inter-POP network needs to be able to guarantee up to full 10 Gbps of available bandwidth between each pair of the POPs when so demanded by the inter-POP traffic load patterns. Therefore, with the dedicated point-to-point link based network architecture, four two-directional 10 Gbps point-to-point links are needed per each POP router to guarantee the required connectivity with the rest of the five POPs. As a result, 5×4=10 protected i.e. total of twenty 10 Gpbs inter-POP connections would be needed. Thus, whereas the inter-POP network serves only 10 Gbps per POP i.e. in total 50 Gbps of service interface capacity, a total of 20×10 Gbps=200 Gbps of physical network capacity is needed to support that service interface capacity. Note also that a network alternative wherein the 10 Gbps (corresponding to a STS-192c POS link) egress packet forwarding capacity pool of each of the POP routers is statically broken into four 2.5 Gbps (STS-48c POS link) connections from each one of the four remote POPs would be blocking up to 10 Gbps-2.5 Gbps=7.5 Gbps worth of billable traffic per POP, thus wasting up to 75% of the traffic forwarding capacity of the POP routers.
Obviously, the dedicated link-based network architecture is the more inefficient the greater the number of sites to be interconnected is. Though the above example assumed the sites to be ISP's POP routers, the sites could equally well be aggregator switches or routers of LECs or other type of communications or applications service providers, or enterprise border routers.
A conventional solution to the above inefficiency and scalability problem of building multi-site interconnect networks based on non-shareable inter-site links is to interconnect the sites through a central packet-switch, instead of directly to each other in a full-mesh fashion. However, in order to not deteriorate the network QoS from the level of direct site-to-site full-mesh, a dedicated central switch is needed for each such client network. Furthermore, in order to avoid the central switch from forming a global single-point-of-failure, each central switch needs to be doubled for protection. It is clear that ensuring QoS via such per-subscriber-dedicated and redundant link and switch based communications networks looses the economical advantage of providing connectivity over multi-subscriber shared public Internet, and thus does not improve the cost-efficiency of packet networking. Rather, such per-client-dedicated networks merely trade off the cost advantage of shared packet-switched Internet to gain better QoS for the client networks.
It should further be understood that the above reasoning applies not only to client-dedicated networks based on per-client-dedicated L1/L0 network connections and switches, but equally well also to packet-layer (L2/L3) virtual private networks (VPNs) that still require reserving network resources at the packet layer for each inter-site connection per each client separately in order to guarantee site-to-site connectivity parameters i.e. provide deterministic QoS. Providing the required QoS per each client network by using shared L1/L0 connections and packet-switches and reserving adequate capacity at the packet layer rather than by using per-client-dedicated connections and switches may enable finer granularity of capacity allocation, but that comes with the cost of significantly more complex and costlier packet-layer network elements. Thus, the L2/L3 VPNs also appear to create another trade-off between performance and cost advantage, and do therefore not produce a clear cost-efficiency improvement.
To summarize, while networks based on conventional dedicated L1 point-to-point circuits are often prohibitively inefficient for bursty packet traffic, conventional L2 (or above) shared packet-switched networks are not able to provide deterministic QoS without compromising the bandwidth efficiency of packet-switching, by reserving capacity for the connections requiring guaranteed throughput, and without making the packet-switched network infrastructure more costlier.
However, as the demand for reliable packet-based communications services is growing, driven in particular by Internet-related applications, communications service providers need to be able to provide deterministic and sufficiently good QoS also for packet-based communications services cost-effectively. There thus is a strong demand for a new technique that enables to provide multiple private-like networks for multi-sited clients using economical, multi-client-shared network infrastructure. Such new technique should provide the performance and QoS of a network based on dedicated L1 point-to-point connections among the sites of each client, with the cost advantage of using packet-layer (L2+) shared network resources, and without increasing the cost of such packet-switched network infrastructure.